This is an apparent GM problem, when the transistor in the ABS unit burns our, the pump will continue to run with the ingition off.. Pull the 60 amp fuse out. Contact http://www.modulemaster.com/en/index.php They have rebuilt units for only 130.00.. Much cheaper than the 950.00 for the GM dealer or any other folks i have found.. Good Luck
ABS kept running after engine shut off and key turned off. Replaced ABS.
about 8 or 9 years ago my dad had this problem. H went to the local GMC dealer. they unplugged it to keep the battery from discharging and turn it into regular breaks They told him it was no longer covered and the fix would be around 800 dollars. It has been that way since. It now has 43,000 miles Sept 2011.